1. Field of the Invention
This invention relates to a process for recovering useful oil from a waxy slurry dumped from a wax filter. More particularly, this relates to an improvement in a DILCHILL dewaxing process wherein waxy slurry containing useful lube oil is periodically dumped from the wax filters and the filter cloth is then washed with hot solvent, wherein the improvement comprises mixing the filter hot washings with the dumped slurry and recycling said mixture back into the DILCHILL dewaxing zone in an amount ranging from about 0.02 to 0.4 volumes of recycle per volume of the waxy feed and at a point therein wherein the temperature of the recycled mixture ranges from about 5.degree. to about 50.degree. F. below the bulk temperature of the oil in said dewaxing zone.
2. Description of the Prior Art
It is well known in the art to dewax wax-containing hydrocarbon oils, particularly the lube oil fractions of petroleum oil, in order to remove at least a portion of the wax therefrom to obtain a dewaxed oil of reduced cloud and pour points. This is done via the use of various solvent dewaxing processes in which the temperature of the wax-containing oil is lowered sufficiently to precipitate wax therefrom as solid crystals of wax. The presence of dewaxing solvent in the oil improves the fluidity and reduces the viscosity of the resulting slurry so that various filtration or centrifugation processes can be used to separate the wax from the dewaxed oil. Perhaps the best solvent dewaxing process in use today is the DILCHILL dewaxing process wherein a waxy oil feed is introduced into a staged chilling (wax crystallization) zone and passed from stage to stage of the zone, while at the same time cold dewaxing solvent is injected or introduced into a plurality of the stages therein and wherein a high degree of agitation is maintained in the stages so as to effect substantially instantaneous mixing of the waxy oil and solvent. As the waxy oil passes from stage to stage of the chilling zone, it is cooled to a temperature sufficiently low to precipitate wax therefrom without incurring the well known shock chilling effect. This process produces a wax/oil/solvent slurry wherein the wax particles have a unique crystal structure which provides superior filtering characteristics such as high filtration rates of the dewaxed oil from the wax and high dewaxed oil yields. This DILCHILL dewaxing process is disclosed in U.S. Pat. No. 3,773,650, the disclosures of which are incorporated herein by reference.
The waxy slurry produced in the DILCHILL dewaxing or wax crystallization zone is passed to scraped-surface chillers and then to continuous rotary drum filters in order to separate the solid wax particles from the dewaxed oil and solvent. Continuous rotary drum filters are well known and used in the petroleum industry for wax filtration and models specifically designed and constructed for filtering wax from fractions such as lube oil fractions are commercially available from manufacturers such as Dorr Oliver and Eimco. A typical rotary drum vacuum filter comprises a horizontal, cylindrical drum, rotatably mounted about its longitudinal axis, the lower portion of which is immersed in a vat containing the wax slurry, a filter surface comprising a cloth covering the horizontal surface of the drum, means for applying both vacuum and pressure thereto and means for both washing and removing wax cake deposited on the cloth as the drum continuously rotates around its horizontal axis. In these filters, the drum is divided into compartments or sections, each section being connected to a rotary (trunion) valve and then to a discharge head. The wax slurry is fed into the filter vat and as the drum rotates, the faces of the sections pass successively through the slurry. In a vacuum filter, a vacuum is applied to the sections as they pass through the slurry, thereby drawing oily filtrate through the filter medium and depositing wax thereon in the form of a cake. As the cake leaves the slurry, it contains oily filtrate which is removed therefrom by the continued application of vacuum, along with wash solvent which is evenly distributed or sprayed on the surface of the cake, thereby forming a solvent-rich wash filtrate. Finally, the wax cake is removed by a scraper which is assisted by means of blow gas applied to the interior of each section of the drum as it rotates and just before it reaches the scraper. In a pressure filter, the solvent contains an autorefrigerant, which, by virtue of its relatively high vapor pressure, is sufficient to apply a pressure differential across the filter surface of the drum, thereby eliminating the need for applying a vacuum thereto.
After a continuous rotary drum wax filter has been operating for a period of time which may vary from two hours to two days, the filter cloth starts to become clogged with very fine wax crystals which reduce the filtration rate. Therefore, it is necessary to periodically stop the filtering action, dump the slurry from the filter vat and apply hot solvent to the filter cloth for a few minutes in order to dissolve the fine wax crystals from the pores of the filter cloth. The filter vat is refilled with slurry and the filtration operation is then resumed. This dumped slurry contains useful dewaxed oil--sometimes amounting to one or two percent of the waxy oil feed going to the DILCHILL dewaxing or wax crystallization zone. It cannot be sent directly to dewaxed oil recovery, because the wax content is too high which would throw the dewaxed oil product off specification on cloud and pour points. The dumped slurry may be recovered as foots oil if there is a wax manufacturing operation, otherwise it goes into slack wax and typically ends up in fuel oil or as cat cracker feed. In any event, it is lost as dewaxed oil if disposed of in this manner.
In an attempt to avoid the loss of dewaxed oil, it has sometimes been the practice to inject the dumped slurry into the outlet of the DILCHILL wax crystallization zone upstream of the scraped surface chillers. Unfortunately, if this is done the partially dissolved wax crystals do not have a chance to grow to the proper size, because they are not exposed to the unique environment in the DILCHILL wax crystallization zone. This results in the presence of fine wax crystals in the wax filter feed which reduces the filtration rate thereby adversely affecting plant throughputs.
Alternatively, if one wishes to feed the dumped slurry at a significantly high rate into the entrance of the DILCHILL chilling zone, along with the waxy feed, it is necessary to heat the cold slurry up to about 130.degree. F. or so in order to avoid shock chilling the feed. Not only does this result in loss of the potential chilling capacity of the dumped slurry, but it also requires additional heat and places additional load on the refrigeration system.
It would therefore be an improvement to the art if a way could be found to recover the useful dewaxed oil from the slurry that is periodically dumped from the filter vat, without adversely affecting either the filtration rate or the refrigeration load.